FIG. 9 is a block diagram of conventional touch key 1001 disclosed in Japanese Patent Laid-Open Publication No. 2003-224459. Electrode 2 is provided on a surface of panel 1 made of an electrical insulator, such as glass, having a flat surface. Electrode 3 facing electrode 2 is provided on the other surface of panel 1. Electrode 2 and 3 and panel 1 provides a capacitor. Oscillator 4 outputs a high-frequency voltage, and outputs the voltage to electrode 3. Oscillator 4 has high output impedance. When finger 9, a conductive object, touches electrode 2, electrode 3 is bypassed through panel 1, electrode 2, and finger 9 to be grounded, thereby reducing the high-frequency voltage at electrode 3 to a voltage lower than that before finger 9 touches electrode 2. Rectifier 5 rectifies and smoothes the high-frequency voltage output from oscillator 4, the high-frequency voltage applied to electrode 3, so as to convert the high-frequency voltage to a direct-current (DC) voltage, and outputs the DC voltage to voltage divider 12. The DC voltage appearing when electrode 2 is not touched is higher than the voltage appearing when electrode 2 is touched. Voltage divider 12 divides the DC voltage output from rectifier 5 and outputs a low DC voltage having an absolute value lower than that of the DC voltage output from rectifier 5 to judging unit 13. When the direct-current voltage input to judging unit 13 changes, judging unit 13 calculates a voltage difference between voltages before and after the change, and determines that finger 9 touches electrode 2 if the voltage difference exceeds a predetermined value. The DC voltage output from rectifier 5 has a high absolute value, and may accordingly break judging unit 13 or causes judging unit 13 to malfunction. In order to prevent this problem, the DC voltage output from rectifier 5 is divided by voltage divider 13 to be lowered to a predetermined level.
FIG. 10 illustrates the high-frequency voltage output from oscillator 4. Finger 9 does not touch electrode 2 before time T0, and continues touching electrode 2 after time T0. Before time T0, finger 9 does not touch electrode 2, hence causing oscillator 4 to output voltage Vosc101. After time T0, finger 9 touches electrode 2, hence causing oscillator 4 to output voltage Vosc102 having an amplitude smaller than that of Vosc101. FIG. 11 illustrates the DC voltage output from rectifier 5. Voltage V10 output when finger 9 does not touch electrode 2 is a DC voltage into which voltage Vosc101 before time T0 shown in FIG. 10 is converted. Voltage V11 output after time T0 when finger 9 touches electrode 2 is a DC voltage into which voltage Vosc102 shown in FIG. 10 is converted. Thus, finger 9 touches electrode 2, thereby producing voltage difference ΔV6 between voltages V10 and V11.
Judging unit 13 generally includes a semiconductor, such as a microprocessor, which is weak to an excessive input, and accordingly, it is important that a voltage input thereto is limited. FIG. 11 shows voltage Vk, an upper limit of the voltage input to judging unit 13. The voltage output from rectifier 5 is higher than upper limit voltage Vk, and hence cannot be input to judging unit 13.
FIG. 12 illustrates the voltage output from voltage divider 12. Voltage divider 12 divides voltages V10 and V11 output from rectifier 5 to output voltages V12 and V13, respectively. Judging unit 13 detects voltage difference ΔV7 between voltages V12 and V13 as to determine that finger 9 touches electrode 2 when voltage difference ΔV7 exceeds a predetermined value.
A cooker including touch key 1001 is operated while a cooked object is being looked at, hence being used easily.
In touch key 1001, voltage difference ΔV7 between voltages V12 and V13 is divided as well as voltages V10 and V11, and thus being smaller than voltage difference ΔV6. Voltage difference ΔV6 is thus small, and accordingly reduces the sensitivity of the judging of judging unit 13 to determine whether finger 9 touches electrode 3 or not.